I would use thicker traces. I can't tell in the photo, but they look very narrow. Which is good for reducing power loss but not so good on rework, as you've found. I also wonder about making the pad shorter--leave it extending past the case for manual soldering, but starve the crystal underneath, in essence making the pad smaller and thus faster to heat up.
I tend to think of design for rework a bit differently; on the ATE boards I do most of the connectors are routed such that I can easily cut traces and rewire, in case of design issues. Extra wireholes on power supplies etc. Different beast though from what you're dealing with.
Shawn Upton, KB1CKT
Test Engineer
Allegro MicroSystems, LLC
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603.626.2429/fax: 603.641.5336
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Carl VanWormer
Sent: Monday, August 19, 2013 1:38 PM
To: [log in to unmask]
Subject: [TN] DFR (Design For Rework) - I'll never have this problem again
I just wasted a couple of hours finding a self-created problem with a 27MHz crystal oscillator rework. The SMT crystals were changed in a batch of 6 prototypes, but one of the units was failing with baffling results, intermittently seeming to almost work . . . The problem was finally discovered as a PCB trace that was broken during rework (by me), but I noticed the reason for the break and a solution to prevent future occurrences of the problem:
The SMT crystal package (HC49/US) has leads that exit the metal case near the center, bending over the plastic base to extend past the ends of the package, allowing visual indication of proper soldering. One of the tiny traces on my board came to the inside end of one of the PCB pads (see http://stevezeva.homestead.com/CrystalRouting.jpg for a picture of the PCB artwork. Thanks, Steve)
Rework of this part is difficult to do with a single soldering iron, heating one end and prying up gently on the package until that end lifts slightly, repeating on the other end, back and forth . . .
Unfortunately, the inside corners of the pads are the last to melt, and have the most mechanical advantage to lever-up the end of the pad. My pad lifted slightly, invisibly tearing the connecting trace.
The solution to this special case combination is to not make electrical contact to the inside end of the pads (under the part, and out of view), but to connect the PCB traces to the outside ends, eliminating this fringe problem. I'm calling this approach DFR (Design For Rework), and am adding this to my PCB checklist.
Later,
Carl (attractor of fringe problems)
Carl Van Wormer, P.E., AE7GD
Senior Hardware Engineer
Cipher Engineering LLC
21195 NW Evergreen Pkwy Ste 209
Hillsboro, OR 97124-7167
503-617-7447x303
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